The value of a cut diamond is substantially determined by the so-called “4-Cs”: Carat, Colour, Clarity and Cut. Of these, carat and colour are substantially objective characteristics that may be assigned specific values in a largely uncontentious manner. The carat value is simply the weight of the diamond, wherein one carat is equal to 200 milligrams. Colour, in terms of both hue and transparency, may also easily be measured objectively, although the results of such measurement are somewhat dependent upon lighting conditions. Most diamonds used as gemstones are substantially transparent with little tint (known as “white diamonds”), however the most common impurity, nitrogen, causes a yellow/brown tint that is present, to some degree, in almost all white diamonds. The most widely-used colour rating system is that of the Gemmological Institute of America (GIA), which assigns a grade from D (colourless) to Z (bright yellow colouration) to nominally white diamonds. Generally, less colouration is more desirable, although diamonds of hues other than yellow/brown, such as pink or blue diamonds, may be more valuable than white diamonds, due to their rarity and/or level of market interest. In any case, assessing diamond hue against colour standards, and transparency in terms of light absorption, presents little practical difficulty.
Clarity and cut, on the other hand, influence the visual appearance of gemstones in a relatively complex manner. Indeed, these characteristics exhibit complex interactions with one another, as well as with lighting, viewing position, and so forth. It has therefore proven far more difficult to arrive at broadly accepted objective measures of clarity and cut, and the measures that do exist do not always accord closely with the subjective beauty of cut gemstones, as assessed by consumers, under a range of everyday lighting and viewing conditions.
Gemstone clarity, in particular, is a quality relating to the existence and visual appearance of internal flaws often called “inclusions”, or “internal characteristics”. Clarity is also affected by surface defects, or blemishes. There are various causes of inclusions, which may be, for example, crystals of a foreign material, another crystal of the gemstone itself, or imperfections such as cracks which may appear whitish or cloudy. The clarity of a gemstone, such as a diamond, will depend upon the number, size, colour, location, orientation and visibility of inclusions.
The most widely used clarity grading scales are those of the GIA and the Hoge Raad voor Diamant or Diamond High Council (HRD), according to which clarity is measured under 10-times magnification with specified “dark field” illumination. This basically involves illuminating the base of the stone from the side, and observing the resulting appearance of the stone from the top under 10-times magnification. The GIA diamond grading scale provides a total of 11 grades in six categories. The categories and grades are: flawless (FL), internally flawless (IF); very very slightly included (VVS1, VVS2); very slightly included (VS1, VS2); slightly included (SI1, SI2); and included (I1, I2, I3). Diamond graders are trained to assign grades from the GIA scale in a consistent manner.
As noted above, complex interactions exist between clarity and cut. In particular, by identifying inclusions within a rough (uncut) gemstone, it is possible, in principle, to plan the cutting of the gemstone in such a manner as to minimise the impact of those inclusions upon the clarity of the final cut and polished stone. This is, however, not a trivial matter. Planning is accordingly one of the most valuable skills in the diamond industry, with rough stones being evaluated from an economic perspective, with a view to maximising the value and saleability of diamonds cut from a rough stone. All else being equal, larger (ie higher carat) diamonds are more valuable. Accordingly, it is generally desirable to cut the largest stones possible from the rough, assuming that this can be done without causing an unacceptable reduction in the clarity of the resulting stone.
Modern diamond planning is facilitated by sophisticated electronic and computerised tools. Scanning devices, often incorporating digital imaging and laser ranging technologies, are used to capture a three-dimensional (3D) computer model of the rough stone prior to cutting. At this stage, images of inclusions may also be captured, and their relative location within the stone estimated. Computer software tools then assist the planner in placing proposed cut stones within the three-dimensional model of the rough stone, and assessing their potential value, prior to the commencement of any actual cutting.
However, there remain considerable opportunities for further improvements to be made in the tools available to assist in the planning and evaluation of gemstones. As noted above, clarity grading is based upon visual appearance, under 10-times magnification and specified lighting and viewing conditions. It should be appreciated that the actual visual appearance of a gemstone under these conditions does not depend only upon the number and/or types of inclusions. In particular, the visual appearance also depends upon the location and orientation of the inclusions relative to the facets of the cut stone. For example, a thin crack viewed edge-on may be barely visible, whereas the same crack viewed side-on may substantially compromise the clarity of the cut stone. Such considerations are significantly complicated by the reflection and refraction of light at facets of the cut gemstone. In particular, the facets of a diamond are designed to act as tiny mirrors, which reflect light back up towards the surface of the diamond, in order to enhance its visual appearance. However, this mirror effect may also act to “multiply” inclusions, so that they may appear more numerous than they really are, and furthermore so that they may be visible, in reflection, from angles other than that directly presented to the viewing position.
Taking all of these complex factors into account, planning is not simply a matter of identifying an optimum compromise between size of a cut stone, and number of inclusions. The more sophisticated software tools therefore endeavour to provide the user with a more accurate representation of the visual appearance of the cut stone, taking into account the optical effects such as reflection and refraction of light. In the case of internal flaws, this may include placing wire-frame or false-colour representations of inclusions within a three-dimensional model such that the actual apparent number and size of the inclusions in the resulting stone may be evaluated on-screen. A further level of sophistication involves the capturing of images (eg photographs) of the inclusions, and placing these photographs on the visual images displayed to the user. While these approaches are useful, they remain inadequate because they fail accurately to show the actual visual appearance of the inclusions in the finished gemstone. Photographs, in particular, are limited to showing the inclusions as they appeared when photographed in the uncut stone, rather than under the specific lighting conditions applied for the purposes of clarity grading. Existing approaches have therefore proven inadequate for performing virtual clarity grading during the planning process, and this aspect of planing therefore remains an imprecise art relying upon the skill of the planner and, inevitably, some degree of intuition, or even luck.
In addition to planning for the purposes of optimising the clarity grade under the GIA grading conditions, it is often also desirable to assess the appearance of an existing cut gemstone, or a proposed gemstone to be cut from a rough stone, under a range of alternative lighting conditions. Notably, the lighting conditions used for GIA clarity grading are not the conditions under which the stone will be viewed by consumers in the showroom, or during normal wear of diamond jewelry. It may therefore be extremely useful to provide improved computer modeling of gemstones with internal flaws for the purposes of “virtual viewing” from a range of viewing positions, and under a variety of lighting conditions. Indeed, while ray-tracing software exists today that is capable of generating virtual images of three-dimensional objects under arbitrary viewing conditions, and even of animating such images, there is presently no effective way to obtain a sufficiently accurate model of a gemstone with internal flaws for use with such software.
It is therefore an object of the present invention to provide for improved virtual modeling of the visual appearance of rough and cut diamonds, for the purposes of, for example, evaluation, planning and virtual viewing.